1. Field of the Invention
The present invention relates to a cathode discharging apparatus, and more particularly, to a hollow cathode discharging apparatus for generating low temperature slender plasma jets.
2. Description of the Prior Art
Hollow Cathode Discharge (HCD) is a common technique for generating low temperature plasma. Referring to FIG. 1, a conventional hollow cathode discharging apparatus 1 comprises a hollow anode electrode 11 and a hollow cathode electrode 13 enclosed by the hollow anode electrode 11. The hollow cathode electrode 13 is connected to a high frequency power generator 15 (for example, a high frequency power generator operable at a frequency of 13.56 MHz) and insulatedly spaced apart from the hollow anode electrode 11 by an insulated pipe 17 made of aluminum oxide ceramic. A reactive gas (for example, argon, helium, and nitrogen) required to generate plasma is introduced into the hollow cathode electrode 13 through an insulated gas pipe 18 penetrating the hollow anode electrode 11. Then, the reactive gas in the hollow cathode electrode 13 is ionized under high-frequency power supplied by the high frequency power generator 15. In the hollow cathode electrode 13, the free electrons collided with the reactive gas and bring high-density plasma. Then, the high-density plasma is ejected from a jet hole 19 to form a plasma jet. The plasma jet can be applied to various processes, such as surface modification and film deposition.
To be useful for a large area film coating process, the aforesaid single plasma jet may be expanded to multi-plasma jet for large area deposition application. Normally, generation of multiple, separate plasma jets is achieved by a conventional hollow cathode discharging apparatus lying horizontally and equipped with many separate reaction chambers each having a nozzle to enable ejection of the plasma, as disclosed in European Patent No. EP0881865.
Patent No. EP0881865 discloses an apparatus for generating a plurality of low temperature slender plasma jets. Referring to FIG. 2, an apparatus 2 comprises a hollow anode electrode 21 and a hollow cathode electrode 22 insulatedly enclosed by and yet spaced apart from the hollow anode electrode 21. The hollow cathode electrode 22 is connected to a high frequency power generator for generating a high-frequency power source, partitioned into a plurality of separate circular reaction chambers 24 by a plurality of partitions 23, and coaxially penetrated by a gas transfer pipe 25 insulated from the hollow anode electrode 21. A reactive gas required to generate plasma is introduced into the circular reaction chambers 24 through the gas inlet pipe 252. The gas transfer pipe 25 is disposed with gas apertures 251 corresponding in position to the circular reaction chambers 24 respectively. The hollow cathode electrode 22 and the hollow anode electrode 21 are disposed with cathode openings 221 and anode jet holes 211 respectively. The cathode openings 221 and the anode jet holes 211 correspond in position to one another. Confined to the circular reaction chambers 24, free electrons vibrate in such a way as to generate high-density plasma. The high-density plasma was ejected from the anode jet holes 211 to form a plurality of separate plasma jets 26. The separate plasma jets 26 are used in film deposition.
The separate circular reaction chambers 24 are separated from one another by the partitions 23 and adapted to provide a plurality of successive plasma jets 26. Given the extremely high density of the individual plasma jets 26, film deposition is performed fast but slowly in between any two. The speed varies greatly. To achieve uniform film thickness, a substrate 27 is kept away from the anode jet holes 211 by a distance d as great as possible. In so doing, the density of plasma decreases undesirably, thus slowing down the film deposition.
Referring to FIGS. 2 and 3, the circular reaction chambers 24 correspond in position to the gas apertures 251 of the gas transfer pipe 25. Inasmuch as plasma is flew to the vacuum chambers by vacuum suction, poor alignment of the gas apertures 251 prevents uniform distribution of the reactive gas inside the circular reaction chambers 24.
With the gas transfer pipe 25 penetrating the hollow cathode electrode 22 axially, an increase in the diameter of the gas transfer pipe 25 is always accompanied by a decrease in the capacity of the circular reaction chambers 24, thus aggravating non-uniform distribution of the reactive gas. For this reason, the diameter of the gas transfer pipe 25 is necessarily small. Scale up the apparatus 2 in axial, coupled with the small diameter of the gas transfer pipe 25 therefore prevents uniform distribution of the reactive gas inside the slender gas transfer pipe 25. Upon its entry into the separate circular reaction chambers 24 individually, the different reactive gas rate has immediately bearing on different plasma density. The longer the apparatus 2 is, the less is uniformity of plasma density in its axial direction. The disadvantage poses hindrance to large-area film deposition.
Accordingly, an issue facing the industrial sector and calling for urgent solution is to develop a hollow cathode discharging apparatus that facilitates scale-up of a HCD apparatus in axial, high density and uniform plasma distribution was induced by uniform distribution of a reactive gas.